German patent DE 26 58 518 C2 describes such apparatus. It shows typical conditions when processing LDPE (Low Density Polyethylene). Just after leaving a ring nozzle, tubing foil expands to its final diameter. An outer blow ring is provided, the nozzles of which are directed diagonally upward onto the area of the expansion. Following a flattening device for the tubing foil, a measuring device is provided with which the thickness flow of the foil is measured. The rate of air flow through the nozzles of the air nozzle ring is controlled dependent on this thickness flow to prevent the formation of so-called barrel hoops on the tubing foil. As a result of cooling by means of the nozzles of the air nozzle ring, a thickening of the foil tubing is attained in the region of the cooling. When heating is used, the foil becomes thinner in the heated area.
When processing HDPE (High Density Polyethylene), one usually works with a long neck which is possible, among other things, due to the expansion viscosity which is higher in comparison to LDPE. It became apparent in this case that the cooling of the tubing was not uniform in this region due to the length of the long neck and due to the resultant differences in temperature which led to differences in the expansion viscosity. These differences in the expansion viscosity disadvantageously influence the foil tolerance.
To counter this, attempts were made to divide the outer blow ring surrounding the ring nozzle into segments in which the rate of air flow could be adjusted. An example of this type of a device is described in German patent DE 37 43 720 C2.
A further possibility for reducing the differences in viscosity is the use of devices which led the cooling air coming out from the outer blow ring into a chamber surrounding the long neck, the chamber being provided with one or more iris diaphragms. However, differences in expansion viscosity in the material also occur with a device of this type.